[4] Since 2004, he has been an independent researcher working in the area of economic and business history, based in Berlin.
[5] Karlsch investigated the four-year history of Nazi German atomic research through a collaboration with the TV journalist Heiko Petermann.
In 2005, he published his controversial book Hitlers Bombe, in which he presented evidence that Nazi scientists working under Kurt Diebner experimented inconclusively with a large atomic reactor during the final stages of World War II and may have tested a crude nuclear weapon in Thuringia on 3 March 1945, killing several hundred prisoners of war and concentration camp inmates.
[6] The results of the research, along with an additional unpublished document from the Russian state archives, were then summarised by Karlsch and a US historian of Nazi science, Mark Walker, in Physics World.
A number of other historians active in the field – Dieter Hoffmann, Paul Lawrence Rose, Bernhard Fulda, and Michael Schaaf – have played down the significance of the details uncovered by Karlsch and disputed that it was possible to speak of a Nazi nuclear test or a Nazi atom bomb.